The present invention relates to detection and reduction of block noises generated when video signals are coded and decoded per pixel block.
A well known encoding technique is to compress video signals per pixel block with exploiting correlation between adjacent pixels within each block. Each pixel block consists of a predetermined number of pixels in the horizontal and the vertical directions.
The coded video signals are stored in a storage medium or transferred to a decoding apparatus via a transfer cable. After storage or transfer, the coded video signals are expanded per pixel block for decoding.
The lower the compression ratio, the higher the image quality. On the other hand, the higher the compression ratio, the smaller the amount of video data stored in a storage medium or transferred along a transfer cable. High compression ratio would however cause a difference in gradation between adjacent pixel blocks. The gradation difference is called a block noise and noticeable on video signal portions with small gradation changes.
Block noises would also be generated when video signals are reproduced from a storage medium by a dirty or worn-out magnetic head. These noises are also noticeable on a monitor.
Several techniques have been developed for detection and reduction of block noises.
One of the techniques is to smooth the boundary between adjacent pixel blocks over which block noises are generated, by interpolation, as shown in FIG. 1.
This figure shows signal levels of four pixels x1 to x4 aligned over the boundary between pixel blocks BK1 and BK2.
The difference in signal level between the pixels x1 and x2 in the block BK1 is d1, and that between x3 and x4 in the block BK2 is d3. The signal difference between the pixels x2 and x3 adjacent to each other over the block boundary is d2. The difference d2 is larger than d1 and d3, thus causing generation of block noises.
The block noise can be detected by comparison of signal level over the block boundary for reduction if the boundary is already known. If not, however, the block boundary must to be detected from an input video signal.
Such noise reduction requires precise boundary detection by, for example, a decoding apparatus capable of outputting pulse signals that indicate pixel block boundaries.
Japanese Unexamined Patent Publication Nos. 1991(3)-174891 and 1996(8)-149470 disclose other techniques for detecting and reducing block noises. These techniques however require a bulk of circuitry.